A third type of spontaneous DNA damage that occurs frequently is damage to the bases by free radicals of oxygen. These arise in cells as a result of oxidative metabolism and also are formed by physical agents such as radiation.. Base pair (nucleotide pair) substitutions. These are of two types: transitions (purine to purine or pyrimidine to pyrimidine) and transversions (purine to pyrimidine or pyrimidine to purine). We break these down into the two categories because they can occur in different ways.
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A flight delay is when an airline flight takes off and/or lands later than its scheduled time. The Federal Aviation Administration (FAA) considers a flight to be delayed when it is 15 minutes later than its scheduled time. A cancellation occurs when the airline does not operate the flight at all for a certain reason.